Offset lithography is a process well known in the art which utilizes the planographic method. Image and non-printing areas are essentially on the same plane of a thin metal plate and the distinction between them is maintained chemically. Ink is offset from a plate on the plate cylinder to a rubber blanket on a blanket cylinder and then from the blanket to a substrate supported on an impression cylinder on which printing occurs.
Conventional sheet-fed, rotary offset printing presses typically include one or more printing units through which individual sheets are fed and printed. After the last printing unit, freshly printed sheets are transferred by a delivery conveyor to the delivery end of the press where they are collected and stacked uniformly. In a typical sheet-fed, rotary offset printing press, the delivery conveyor includes endless chains carrying gripper bars with gripper fingers which grip and pull freshly printed sheets from the last impression cylinder and convey them to the sheet delivery stacker.
Printed lithographic ink on the surface of the substrate sheet dries relatively slowly through oxidation and is easily smeared by subsequent transfer cylinders between the individual printing units of the press. Any relative movement of the freshly printed surface relative to a support surface can result in smearing. Modified and specialized equipment and techniques have been developed to combat this problem.
A related problem that is faced in the prior art is the problem of “offsetting” and “set off” of freshly printed ink at the delivery end of the press after the printed sheets are collected and stacked. A similar problem occurs in roll form material produced on a web-fed press. In some printing jobs, offsetting is prevented by applying a protective and/or decorative coating material over all or a portion of the freshly printed sheets. Some coatings are formed of an ultra-violet (UV)-curable or water-dispersed resin applied as a liquid solution over the freshly printed sheets to protect the ink from offsetting or set-off and improve the appearance of the freshly printed sheets. Such coatings are particularly desirable when decorative or protective finishes are applied in the printing of posters, record jackets, brochures, magazines, folding cartons and the like. In cases where coating is to be applied, the coating operation may be carried out after the last printing unit, most desirably by an in-line coating application. It is highly undesirable to process the sheet through the press a second time in order to apply coatings, although this is sometimes done for special effects that are not otherwise obtainable.
The ability to overall coat, spot coat or print with aqueous, flexographic and UV curable inks and/or coatings in combination with lithographic, flexographic and waterless printing processes on a rotary offset printing press is highly desirable. Flexographic printing or coating with aqueous, flexographic and UV curable inks from a blanket or a relief plate can permit much heavier wet and dried ink film layers on the substrate. This is largely due to the nature of lithographic inks. Lithographic inks are generally oil based inks that are formulated to print from planographic surfaces based on the principle that oil and water do not mix. Lithographic inks are generally very strong in color value to compensate for the lesser amount that is printed. They are among the strongest of all inks. The average amount of ink transferred to the paper is further diluted by the double split of the ink film between the plate cylinder and the blanket cylinder and between the blanket cylinder and the substrate to be printed in the nip between the blanket cylinder and the impression cylinder. In many situations, only a quarter of the film thickness on the plate is transferred to the substrate. This can make it difficult to obtain sufficient opacity with white or metallic (gold, silver or other metallic) ink or in printing specialized vehicles such as “scratch-and-sniff” materials from a slurry containing encapsulated essence. This often means that sheets or substrate must be removed and transferred to a second type of machine using the flexographic process to apply greater amounts of ink thickness or the sheets must make successive or two or more passes on a lithographic press to achieve desired print quality.
The prior art has attempted to solve these problems to obtain higher applied film weights on lithographic printing presses in a variety of ways. Much of the prior art has disadvantages. Retrofitting existing presses is often difficult because of space considerations, especially between printing units. A dedicated coating unit is often not possible because of limited space and involves press downtime and substantial capital costs. Retrofitted devices that utilize the print cylinder or blanket cylinder of the press can limit the ability of that station to lithographically print in the normal manner.
Coaters which utilize the plate cylinder or the blanket cylinder of the printing unit still suffer from the disadvantage that the coating is split which reduces the wet film thickness that can be applied to the substrate itself. A few add on coating units that print directly on the substrate on the impression cylinder or a transfer cylinder are limited to the last printing station on the press where there is more room for installation. Such equipment can be moved away or the operator can do the make ready work on the opposite side of the last printing station in the conventional work space for the operator. If such equipment is mounted in the interstation space on a lithographic press, the equipment interferes with operator access to the next station.
Much of the prior art consumes large areas of space on the press, both between printing units and in some instances in the overhead area because of the complexity and size of equipment, limited locations are available for which it can be used. Additionally, the prior art devices are heavy; thus, when installing these devices, cranes or similar equipment are often required to properly mount the devices in position. A further disadvantage is that these devices are expensive to manufacture and maintain. Finally, the prior art devices are not designed as portable devices for placement on different printing presses or on different printing units. Most printer coater devices are attached to a single printing unit and require extensive connections that must be disconnected requiring extensive labor and costs. Also, as stated previously, with some embodiments, each time a printer coater is moved, a crane or other transport device is required to remove and carry the printer coater to a different printing unit.
It is desirable not to have to cut into press frame to gain access to the main gears and not to have to manually engage and disengage indexed gear teeth of gears on the coater with gears on the press. The ability to flexographically coat, spot coat or print on the substrate at an intermediate printing station with an apparatus that is inexpensive and compact so that it can fit into small areas is highly desirable. It is also desirable to have a lightweight and portable device so that it can be carried by humans for use on any printing unit of a lithographic printing press or to a completely different printing press of the same size and installed or removed without the use of heavy equipment. In various embodiments, the device of the present disclosure may be able to provide various of these desirable results or even combinations of these desirable results.